Graded resolution field of view ct scanner

ABSTRACT

A CT scanner comprising: a rotor rotatable about an axis of rotation: an X-ray source mounted to the rotor having a focal spot from which an X-ray beam emanates; an X-ray detector array comprising a plurality of X-ray detectors for detecting X-rays in the X-ray beam; wherein the detector array has at least one high resolution region in which detectors have a high packing density and at least one low resolution region in which detectors have a low packing density and are separated by X-ray insensitive regions substantially larger than insensitive regions resulting from septa between detectors that function to reduce detector cross talk.

RELATED APPLICATIONS

The present application claims benefit under USC 119(e) of U.S.Application No. 60/685,418 filed on May 31, 2005 entitled “CT Scannerand Method for Medical Imaging, Particularly Useful for Imaging of theHuman Heart”, the disclosure of which is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to computerized tomography (CT) X-rayimaging and to CT scanners having fields of view (FOVs) adapted forimaging limited volumes of tissue within a patient's body.

BACKGROUND OF THE INVENTION

In CT X-ray imaging of a patient, X-rays are used to image internalstructure and features of a region, an “imaging region”, of thepatient's body. The imaging is performed by a CT scanner comprising anX-ray source that provides an X-ray beam and an array of closely spacedX-ray detectors facing a region, referred to as a “focal spot”, of theX-ray source from which the beam emanates. The X-ray beam is asubstantially “fan-shaped” X-ray beam if the scanner acquires at anygiven time data for imagining only a single “slice” or a small number ofslices of the imaging region of a patient. The beam is a “cone-shaped”X-ray beam if the scanner acquires data for imaging a relatively largenumber, generally more than about 10, slices of a patient's imagingregion. A scanner comprising a fan beam is generally referred to as asingle slice scanner and a scanner comprising a cone beam is generallyreferred to as a multislice scanner. Many of the modern day scanners aremultislice scanners and the discussion hereinafter generally refers tomultislice scanners with obvious modifications where necessary toencompass single slice scanners.

The X-ray source and array of detectors are mounted in a gantry so thatan imaging region of a person being imaged with the CT scanner can bepositioned in a field of view (FOV) of the scanner that is locatedwithin the gantry between the scanner's X-ray source and detector array.When the imaging region of the patient's body is located in the FOV, theX-ray source is operable to provide X-rays that pass through the imagingregion and are incident on the detectors. The patient is usuallysupported lying on a couch that is moveable axially along an axis,conventionally the “z-axis” of a Cartesian coordinate system, relativeto the gantry to position and move the patient axially through the FOV.The X-ray source is rotatable around the z-axis. The size and locationof the FOV is defined by a largest circle in a plane perpendicular tothe z-axis that has its center on the z-axis and for which trajectoriesof X-rays from the X-ray source that are detectable by the detectorarray are substantially tangent to the circle. For convenience ofpresentation and visualization a circularly cylindrical region withinthe X-ray beam having its axis coincident with the z-axis and a crosssection coincident with the largest circle is referred to as a CTscanner's FOV.

In many multislice CT scanners the detectors in the detector array aregenerally configured in rows and columns of detectors positioned on acircularly cylindrical surface having an axis that is parallel to thez-axis and passes through the X-ray source focal spot. Conventionally,the columns are parallel to the z-axis and the rows lie along arcs ofcircles that are perpendicular to the z-axis. Features of the cone beamand detector array are conveniently located with respect to a sagittalplane and a transverse plane. The sagittal plane is a plane thatcontains the z-axis and passes through the X-ray source focal spot. Thetransverse plane is a plane that passes through the X-ray source focalspot and is perpendicular to the z-axis. A location of a given column isconveniently defined by an azimuth angle. The azimuth angle is an anglethat a plane containing the column and passing through the focal spotmakes with the sagittal plane. A given row is conveniently located by adeclination angle. A declination angle is an angle that a planecontaining the row and passing through the focal spot makes with thetransverse plane. The dimensions of the cone beam and detector array aregenerally matched so that X-rays from the X-ray source are substantiallyconfined within a solid angle subtended by the detector array at thefocal spot.

To image features and organs in an imaging region of a patient, thecouch supporting the patient is moved relative to the gantry along thez-axis to translate the patient's imaging region through the scanner'sFOV. As the imaging region moves through the FOV the X-ray source isrotated around the z-axis to illuminate thin “slices” of the imagingregion that are substantially perpendicular to the z-axis with X-rays ata plurality of different view angles. At each view angle and differentaxial positions along the z-axis of the imaging region, detectors in thearray of detectors measure intensity of X-rays from the X-ray sourcethat pass through slices of the imaging region. The intensity of X-raysmeasured by a given detector in the array of detectors is a function ofan amount by which X-rays are attenuated by material in a slice of theimaging region along a path length, hereinafter “attenuation path”, fromthe X-ray source, through the imaging region slice to the givendetector. The measurement provides information on composition anddensity of tissue in the imaging region slice along the attenuationpath.

In some CT scanners an axial scan of a patient is performed in which thepatient is moved stepwise along the z-axis to “step” the imaging regionthrough the FOV. Following each step, the X-ray source is rotatedthrough 360 degrees or about 180 degrees to acquire attenuationmeasurements for slices in the imaging region. In some CT scanners a“spiral scan” of a patient is performed in which the patient is steadilytranslated through the gantry while the X-ray source simultaneouslyrotates around the patient and attenuation measurements for slices inthe region are acquired “on the fly”.

The attenuation measurements for slices of an imaging region of apatient provided by the detectors in an axial or spiral scan aregenerally processed using CT reconstruction algorithms known in the artas filtered back projection algorithms to map the absorption coefficientof the imaging region as a function of position. The map is used todisplay and identify internal organs and features of the imaging region.

CT image reconstruction algorithms are used to process attenuation dataassuming that for each slice in the imaging region and for each voxel ofthe slice, attenuation data is acquired for each view angle for anattenuation path that passes through the voxel. To satisfy thisassumption the FOV of a CT scanner used to image a patient's imagingregion is generally configured sufficiently large to encompass the fullwidth of the patient's body at the imaging region. If portions of animaging region of a patient cannot fit inside the FOV for all viewangles, attenuation data is generally incomplete and artifacts may begenerated in images reconstructed from the data. As a result, detectorarrays of conventional CT scanners are relatively large and compriserelatively large numbers of detectors.

Typically, a row of detectors in a multislice CT scanner detector arrayhas between 700-1,000 detectors and there may be as many as 64 rows ofdetectors in the array so that a detector array in a typical CT scannermay have as many as 64,000 X-ray detectors. Future CT scanners areexpected to have even larger numbers of detectors. The large number ofdetectors requires an extensive electronic support infrastructure forsignal processing and data transfer. The detectors also requirecomplicated mechanical support systems that are configured to hightolerances that provide in addition to mechanical support, various otherfunctions such as radiation collimation and shielding for electronicsassociated with the detectors. As a result, CT scanning systems arerelatively complicated and expensive.

CT scanning can involve exposing a large part of a patient's body topotentially damaging X-ray radiation. Damage from exposure to X-rayradiation is thought to be cumulative and for safety and health reasonsit is desirable to minimize a patient's exposure to X-ray radiationduring a CT scan. For some applications for which a region of interestin a patient's body is a relatively localized region inside an imagingregion it is possible to limit radiation by limiting radiation exposureto parts of the body that are outside of the localized region ofinterest. For example, U.S. Pat. No. 6,385,278, the disclosure of whichis incorporated herein by reference, provides a method of reducingexposure of a patient to X-rays during a CT scan when a region ofinterest to be imaged in a patient's body is a relatively small region,such as the heart. The patent describes collimating the beam so thattissue in the patient's body outside the heart receives less radiationthan heart tissue.

SUMMARY OF THE INVENTION

An aspect of some embodiments of the present invention relates toproviding a CT scanner having a FOV sufficient to encompass the width ofa patient's body that is simpler and relatively inexpensive to producein comparison with prior art CT scanners, yet provides CT images havingimage quality satisfactory for medical applications.

In accordance with an aspect of some embodiments of the invention, theCT scanner comprises an array of X-ray detectors comprising at least onerelatively high resolution region and at least one relatively lowresolution region.

The at least one high resolution region comprises X-ray detectors thatare relatively densely packed per unit area of the region and areseparated one from the other substantially only by thin septa thatfunction to reduce cross talk between adjacent detectors and optionallyby anti-scatter shielding that collimates X-rays. The at least one lowresolution region comprises detectors having a relatively low packingdensity per unit area of the region. In some embodiments of theinvention the low density of the low resolution region is provided, atleast in part by configuring X-ray detectors so that they are separatedone from the other by “dead” areas insensitive to X-rays that aresubstantially larger than dead areas generated by septa typically usedin CT scanners. Optionally, X-ray detectors in the low resolutionregions are separated by dead areas having areas equal to or greaterthan a size of the X-ray detectors. In some embodiments of theinvention, the low density is provided, at least in part, by configuringthe detector array so that at least some X-ray detectors at relativelylarge absolute value azimuth angles are larger than detectors at smallerabsolute value azimuth angles. Optionally at least some of therelatively large detectors at large absolute value azimuth angles areseparated by “typical” septa.

Optionally, the at least one high resolution region comprises a single“central” high resolution region having columns of detectors located atrelatively small absolute value azimuth angles flanked on at least oneside by a low resolution “wing” region. Optionally, the central highresolution region is flanked on both sides by low resolution wingregions.

In some embodiments of the invention, the wings on either side of thecentral high resolution region have a same area. Optionally, the wingsare mirror images of each other. Optionally, areas that are dead areasin one of the wings are homologous to areas of the other wing thatcomprise an X-ray detector. Optionally, all the detectors in thedetector array have a same size. Optionally, detectors in the detectorarray have different sizes. For example, detectors in low detectordensity regions of the detector array may be larger than detectors inhigh density regions of the detector array.

The central high resolution region of the detector array provides higherresolution attenuation data for a portion of a patient's imaging regionthat is illuminated with X-rays that are incident on detectors in thecentral region than for a portion that is illuminated with X-raysincident on detectors in the wings. The portion of the imaging regionfor which higher resolution attenuation data is acquired passes througha central cylindrical region, hereinafter a high resolution FOV(HR-FOV), of the scanner's FOV when the patient is scanned. Thecylindrical region of the HR-FOV is concentric with the cylindricalregion that demarks the FOV. A region of the FOV between the centralhigh resolution HR-FOV and the outer boundary of the FOV is referred toas a “low resolution FOV” (LR-FOV).

To provide a set of attenuation data suitable for processing inaccordance with various back projection algorithms to image the imagingregion, data provided by detectors in the wings, i.e. data acquired fromthe LR-FOV, is processed, optionally by normalization and interpolation,to generate approximations to attenuation data that would be expectedwere the wings to have a resolution the same as the resolution of thehigh resolution region. The processed low resolution data is combinedwith the high resolution data to provide the complete attenuation dataset. The inventors expect that the complete data set can be processed toprovide an image of a localized region of interest (LROI) in a patient'sbody that passes through the HR-FOV that has a resolution satisfactoryfor many medical applications. For example, the heart, spine or liver ofa patient may be imaged with satisfactory resolution using a CT scannerin accordance with an embodiment of the invention.

In accordance with an aspect of some embodiments of the invention thecouch of the CT scanner is translatable horizontally and/or verticallyto center a localized region of interest in a patient's body in theHR-FOV.

In accordance with an aspect of some embodiments of the invention, theX-ray source of the CT scanner is collimated so that X-rays from thesource illuminate substantially only those regions of the detector arraythat are sensitive to X-rays. The collimating shielding results in asubstantial decrease in exposure of a patient to X-rays during aCT-scan.

In some embodiment of the invention, shielding is used to decreasesintensity of X-rays from the X-ray source that illuminate the at leastone low resolution region of the detector array. For example, for adetector array having a high resolution region flanked by two lowresolution wings, a “butterfly filter” having a smoothly varyingthickness that increases with azimuth angle may be used to attenuateX-rays that illuminate the wings. Optionally, uniform thickness “platefilters” may be used to attenuate X-rays that illuminate the wings.

As a result of the configuration of detectors in an X-ray detector arrayin accordance with an embodiment of the invention, a number of detectorsin the array is substantially reduced in comparison to prior art CTscanners having a same FOV. A CT scanner in accordance with anembodiment of the invention therefore requires less intricate andcomplicated mechanical and electronic support structures and may beproduced relatively inexpensively. It is expected that CT scanners inaccordance with embodiments of the invention will be affordable by alarger number of medical centers than the number of medical centers thatcan afford prior art CT scanners. It is also expected that individualmedical centers will be able to afford a greater number of CT scannersin accordance with an embodiment of the invention than a number of priorart CT scanners that they can afford. CT scanners in accordance with anembodiment of the present invention are therefore expected to make thebenefits of CT scanning available to an increased population ofpatients. In addition, the benefits will be able to be provided at alower dose of radiation exposure to patients.

There is therefore provided in accordance with an embodiment of theinvention, A CT scanner comprising: a rotor rotatable about an axis ofrotation; an X-ray source mounted to the rotor having a focal spot fromwhich an X-ray beam emanates; an X-ray detector array comprising aplurality of X-ray detectors for detecting X-rays in the X-ray beam;wherein the detector array has at least one high resolution region inwhich detectors have a high packing density and at least one lowresolution region in which detectors have a low packing density and areseparated by X-ray insensitive regions substantially larger thaninsensitive regions resulting from septa between detectors that functionto reduce detector cross talk.

In some embodiments of the invention, the at least one high resolutionregion comprises a single high resolution region. Optionally, the highresolution region is flanked on one side by a single low resolutionarea.

In some embodiments of the invention, the high resolution region isflanked on both sides by a low resolution region. Optionally, the lowresolution regions have a same size. Optionally, the detectors in thelow resolution regions are arrayed in mirror image configurations.

In some embodiments of the invention, the detectors in the lowresolution regions are arrayed so that areas of one of the lowresolution regions that have detectors are homologous with areas of theother low resolution region that do not have detectors.

In some embodiments of the invention, adjacent detectors in a lowresolution region of the detector array are separated by a distancegreater than 0.3 mm. In some embodiments of the invention, adjacentdetectors in a low resolution region of the detector array are separatedby insensitive regions having areas greater than or equal to about thesize of the X-ray detectors.

There is further provided in accordance with an embodiment of theinvention a CT scanner comprising: a rotor rotatable about an axis ofrotation; an X-ray source mounted to the rotor having a focal spot fromwhich an X-ray beam emanates; an X-ray detector array comprising aplurality of X-ray detectors for detecting X-rays in the X-ray beamhaving at least one high resolution region in which detectors have ahigh packing density and at least one low resolution region in whichdetectors have a low packing density; wherein size of the detectors is afunction of distance from a sagittal plane that passes through the focalspot and axis of rotation. Optionally, detectors farther from thesagittal plane are larger than detectors closer to the plane.

In some embodiments of the invention, the X-ray source and detectorarray define a field of view (FOV) for the scanner having a highresolution portion and a low resolution portion, wherein relatively highresolution attenuation data is acquired for a region of a patient imagedby the CT scanner that passes through the high resolution portion andrelatively low resolution attenuation data is acquired for a region ofthe patient that passes through the low resolution region of the FOV.

Optionally, the high resolution region is defined by an area within afirst circle having a first diameter and the low resolution portion isdefined by an area between the first circle and a second circle having asecond diameter that is concentric with the first circle. Optionally,the first circle has a diameter less than or equal to about 25 cm. Insome embodiments of the invention, the second circle has a diameter lessthan or equal to about 50 cm.

In some embodiments of the invention, the scanner comprises an X-rayradiation shielding that attenuates X-rays provided by the X-ray source.Optionally, the radiation shielding attenuates X-rays that illuminatethe at least one low density region. Additionally or alternatively theradiation shielding optionally comprises plates of absorbing materialhaving substantially uniform thickness. In some embodiments of theinvention, the shielding comprises shielding having a smoothly varyingthickness that increases with displacement to either side of a sagittalplane that passes through the focal spot and axis of rotation.Optionally, the shielding is a “butterfly filter”. In some embodimentsof the invention, the CT scanner comprises X-ray radiation shieldingthat collimates X-rays from the X-ray source so that insensitive areasof the detector array are substantially not illuminated by X-rays.

In some embodiments of the invention, the CT scanner comprises aprocessor that processes attenuation data acquired with the detectorarray and calibrates data acquired responsive to X-rays attenuated bythe shielding to data acquired responsive to X-rays that are notattenuated by the shielding.

In some embodiments of the invention, the CT scanner comprises aprocessor that processes data for a region of a body imaged by thescanner acquired by the low resolution detector region to estimate datathat would be acquired for the body region were the low resolutiondetector region to have a resolution substantially equal to that of thehigh resolution detector region. Optionally, the data acquired by thelow resolution data is processed using an interpolation algorithm.

In some embodiments of the invention, the detectors are arrayed in rowsand columns. In some embodiments of the invention, the CT scanner is asingle slice scanner. In some embodiments of the invention, the CTscanner is a multislice scanner.

BRIEF DESCRIPTION OF FIGURES

Non-limiting examples of embodiments of the present invention aredescribed below with reference to figures attached hereto. In thefigures, identical structures, elements or parts that appear in morethan one figure are generally labeled with a same numeral in all thefigures in which they appear. Dimensions of components and featuresshown in the figures are chosen for convenience and clarity ofpresentation and are not necessarily shown to scale. The figures arelisted below.

FIG. 1 schematically shows a multislice CT scanner for imaging patients,in accordance with prior art;

FIG. 2A schematically shows a multislice CT scanner for imagingpatients, in accordance with an embodiment of the present invention;

FIG. 2B schematically shows an enlarged view of the detector array andcone beam of the CT scanner shown in FIG. 2A, in accordance with anembodiment of the invention;

FIG. 2C schematically shows an enlarged view of another detector array,in accordance with an embodiment of the invention;

FIGS. 3A and 3B schematically show a patient being imaged with a CTscanner, in accordance with an embodiment of the invention; and

FIGS. 4A and 4B schematically show an X-ray cone beam shielded to reduceX-ray exposure of a patient during imaging, in accordance with anembodiment of the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 schematically shows a multislice CT scanner 20 for imagingpatients, in accordance with prior art. In FIG. 1 a patient 22 having animaging region indicated by dashed lines 23 and a localized region ofinterest LROI 24 within the imaging region shown in dashed lines is tobe imaged with scanner 20. LROI 24 is by way of example, the patient'sheart. Only features of multislice scanner 20 germane to the discussionof the present invention are shown in FIG. 1.

Multislice scanner 20 comprises a detector array 26 of X-ray detectors32 and an X-ray source 34 that provides a cone beam of X-rays shown indashed lines 38 for illuminating patient 22 with X-rays. Cone beam 38emanates from a focal spot 35 of X-ray source 34. X-ray source 34 anddetector array 26 are mounted to a rotor 40 of a gantry 42. Rotor 40 isrotatable around the z-axis of a coordinate system 44.

Detectors 32 in detector array 26 are configured in rows 28 and columns29 of detectors 32 so that each detector 32 substantially faces focalspot 35 of X-ray detector 34. Columns 29 are parallel to the z-axis anddetector rows 28 are arcs of circles having their centers locatedsubstantially at focal spot 35 and their planes substantiallyperpendicular to the z-axis. By way of example, and for convenience ofpresentation, detector array 26 is schematically shown comprising arelatively small number of rows and columns 28 and 29 of detectors 32and a relatively small number of the detectors. A modern multislice CTscanner may, by way of example, comprise as many as 64 rows and 1000columns of detectors 32 and many tens of thousands of X-ray detectors 32and CT scanners and even larger numbers of X-ray detectors are possible.

Cone beam 38 has a fan shaped cross section in a transverse plane 37that passes through focal spot 35 and is perpendicular to the z-axis. Anopening angle Θ of the fan shaped cross section is referred to as thefan angle of cone beam 38. The cone beam has an angular extent Φ in asagittal plane 39 that contains the z-axis and passes through focal spot35. Cone beam 38 and detector array 26 are matched so that X-rays in thecone beam illuminate substantially all and substantially only detectorarray 26.

A location of a given column 29 of detectors in array 26 is indicated byan azimuth angle θ that a plane containing the column and passingthrough focal spot 35 makes with sagittal plane 39. An azimuth angle θfor and a plane 41 that together with sagittal plane 39 defines theazimuth angle for one of rows 29 is shown in FIG. 1. Letcounterclockwise and clockwise azimuth angles, as seen looking along thez-axis to the origin of coordinate system 44, be respectively positiveand negative azimuth angles. Then extreme detector columns 29 atopposite ends of array 26 are usually located at azimuth angles equal to+Θ/2 and −Θ/2. (For simplicity it is assumed that cone beam 38 issymmetric relative to transverse plane 37 and sagittal plane 39,although asymmetric beam configurations can be used on CT scanners aswell). Similarly, let position of a given detector row 28 be indicatedby a declination angle φ relative to transverse plane 37 andcounterclockwise and clockwise declination angles as seen looking alongthe positive x-axis to the origin of coordinate system 44 be positiveand negative declination angles respectively. The extreme rows 28 ofdetectors 32 are usually located at declination angles φ equal to +Φ/2and −Φ/2.

A field of view 52, i.e. FOV 52, of scanner 20 is located between X-raysource 34 and detector array 26 and is indicated by dashed lines. Fieldof view 52 is a substantially cylindrical region having an axiscoincident with the z-axis and size determined by fan and cone beamangles Θ and Φ and distance of focal spot 35 from the z-axis. FOV 52 islarge enough so that width of patient 22 is fully contained within theFOV and the FOV typically has a diameter equal to about 50 cm. Patient22 is supported on a couch 46 during imaging of the patient. Couch 46 iscontrollable to be translated axially along the z-axis to move patient22 and imaging region 23 through FOV 52. A controller (not shown)controls motion of couch 46, rotor 40 and intensity of X-rays providedby X-ray source 34.

Multislice scanner 20 can generally be operated in an axial mode or in ahelical mode to image imaging region 23 of patient 22. In an axial modecontroller 48 controls motion of couch 46 to step patient 22 along thez-axis and move imaging region 23 through FOV 52 of scanner 20.Following each step, rotor 40 rotates around the z-axis to rotatedetector array 26, X-ray source 34 and its cone beam 38 around patient22 so as to acquire attenuation measurements of imaging region 23 from aplurality of view angles. In a helical mode, patient 22 is movedcontinuously along the z-axis through FOV 52 as rotor 40 simultaneously,continuously rotates around the z-axis to acquire attenuationmeasurements of imaging region 23 at different view angles. In FIG. 1X-ray source 34 and detector array 26 are shown oriented along thevertical at a view angle optionally referred to as a 0° view angle.

Attenuation data acquired for imaging region 23 is processed using anyof various back projection algorithms to provide an image of internaltissue and organs in imaging region 23 and in particular an image oflocalized region of interest LROI 24, i.e. the heart, of patient 22. Itis noted that to acquire the image of the heart, detector array 26 iscompletely filled with detectors 32 and is used to acquire substantiallysame resolution data for the entire volume of imaging region 23.

FIG. 2A schematically shows a multislice CT scanner 120 and patient 122(partially shown) who is to be imaged using the scanner, in accordancewith an embodiment of the present invention.

Multislice CT scanner 120 comprises many of the components and featuresof CT scanner 20 shown in FIG. 1 and is assumed to have a cone beam 38having a fan angle Θ and cone angle Φ and an FOV 152 having a same sizeas FOV 52 of CT scanner 20. However, CT scanner 120 comprises an array126 of rows 128 and columns 129 of X-ray detectors 132 having high andlow resolution regions that generate internal structure in FOV 152 thatis absent in prior art FOV 52. FIG. 2B schematically shows part ofscanner 120 enlarged for convenience of presentation of details of conebeam 38 and detector array 126.

In accordance with an embodiment of the invention, detector array 126comprises a central high resolution region 140 having a relatively highpacking density of X-ray detectors 132 and optionally two low resolutionwing regions 141 and 142 for which packing density of the X-raydetectors is relatively low.

Any of various conventional spatial configurations of X-ray detectorsand detector sizes may be used to configure detectors 132 in detectorarray 126. Optionally, central region 140 is symmetric with respect tosagittal plane 39 and is bounded by detector rows 132 having azimuthalangular positions θ* and −θ*. A central portion of cone beam 38indicated by dashed lines 144 that illuminates central region 140therefore has an azimuthal extent, i.e. a fan angle, equal to 2θ*. Sideportions 145 and 146 of cone beam 38 illuminate wings 141 and 142respectively of detector array 126. Side portions 145 and 146 spanazimuth angles from θ* to 0.5Θ and −θ* to −0.5Θ respectively and haveazimuthal extents equal to (0.5Θ−θ*) (FIG. 2B). Various differentspatial distributions of X-ray detectors and detector sizes may be usedto configure X-ray detectors in “sparsely populated” wings 141 and 142of detector array 126. In accordance with an embodiment of theinvention, detectors 132 in wings 141 and 142 are separated byrelatively large dead areas 150 that are insensitive to X-rays. The deadareas are substantially larger than dead areas generated by septatypically used in CT scanners to separate detectors in a CT scannerdetector array and reduce cross talk. Septa including optionalanti-scattering shielding foils typically have thickness in a range fromabout 0.05 millimeters to about 0.3 millimeters.

Optionally, X-ray detectors in wings 141 and 142 are separated by deadareas having areas equal to or greater than a size of the X-raydetectors. Optionally X-ray detectors in wings 141 and 142 are separatedby a distance equal to or greater than 0.25 of a largest dimension thatcharacterizes a sensitive area of an X-ray detector in the array.Optionally the detectors are separated by a distance equal to or greaterthan 0.5 of a largest dimension that characterizes a sensitive area ofan X-ray detector in the array. Optionally the detectors are separatedby a distance equal to or greater than 0.75 of a largest dimension thatcharacterizes a sensitive area of an X-ray detector in the array. By wayof example, all detectors 132 in detector array 126 have a same size.

Optionally, the respective configurations of X-ray detectors in wings141 and 142 are mirror images of each other. Optionally, configurationsof X-ray detectors in wings 141 and 142 are such that regions of one ofthe wings that have detectors are homologous with regions of the otherwing that do not have detectors. “Complimentary” wing configurations forwhich homologous regions are “negatives” of each other can beadvantageous. For a same total number of X-ray detectors “negative”detector configurations can provide data from more regions of an imagingregion than mirror image detector configurations.

Whereas detectors 132 in central region 140 and wings 141 and 142 areschematically shown as having a same size and shape, detectors indifferent regions of detector array 126 may have different sizes and/orshapes. For example, detectors 132 in wing regions 141 and 142 may belarger or smaller than detectors in central region 140. Nor do detectorsin a same region, such as a wing 141 or 142 have to have a same sizeand/or shape. In addition, detectors 132 in wings 141 and 142 do notnecessarily lie in rows defined by rows 128 of detectors 132 in centralregion 140.

FIG. 2C schematically shows a detector array 127 in accordance with anembodiment of the invention having a configuration of detectors 132different from that shown in FIGS. 2A and 2B. Detectors 132 in lowresolution wing regions 141 and 142 are substantially contiguous witheach other except for septa but are substantially larger than detectors132 in central high resolution region 140. The relative low detectordensity in wings 141 and 142 is achieved as a result of the relativelarge size of detectors 132 in the wing regions. In array 127 by way ofexample, detector size in wings 141 and 142 increase with increase inthe absolute value of the azimuth angle of detectors in the wings, i.e.as distance of the detectors from central detector region 140 increases.

It is noted that whereas in the above discussion, detector arrays 126and 127 (FIGS. 2B and 2C respectively) are symmetric and have two wings141 and 142 a CT scanner in accordance with an embodiment of theinvention may have an asymmetric cone beam and/or detector array. Forexample, a CT scanner in accordance with an embodiment of the inventionmay have a detector array comprising central region 140 but only one ofwings 141 and 142 and a cone beam shielded to match the shape of theasymmetric detector array. It is also noted that whereas in the abovediscussion detector arrays 126 and 127 are assumed to lie on acontinuous, circularly cylindrical surface, detector arrays, inaccordance with an embodiment of the invention may be formed using anyof various techniques and methods known in the art and are not limitedto circularly cylindrical arrays. For example, a detector array inaccordance with an embodiment of the invention may be configured usingat least one flat panel array.

Central region 140 of detector array 126 and its corresponding centralportion 144 of cone beam 38 define a high resolution region HR-FOV 161of FOV 152. Side regions 141 and 142 of detector array 126 andcorresponding side portions 145 and 146 of cone beam 38 define a lowresolution region LR-FOV 162 of FOV 152. Absorption of X-rays is sampledat a higher resolution for a portion, hereinafter referred to as a“core”, of an imaging region of a patient that passes through HR-FOV 161during a scan of the patient than for a portion, hereinafter a“periphery”, of the imaging region that passes through LR-FOV 162.

In an embodiment of the invention, cone beam 38 and detector array 126are configured so that FOV 152 has a diameter sufficient to encompass atypical width of a patient imaged with CT scanner 120 and HR-FOV 161 hasa diameter sufficient to encompass local regions of interest, i.e.LROIs, such as the spine, heart or liver, within the patient's body. Forexample, FOV 152 may have a diameter of about 50 cm and HR-FOV 161 adiameter of about 25 cm. Other diameters for FOV 152 and HR-FOV 161 arepossible and can be advantageous. For example, in a children's clinicthe FOV and HR-FOV of a CT scanner in accordance with an embodiment ofthe invention may have dimensions that are smaller than those notedabove.

In accordance with an embodiment of the invention, the relativelysparsely sampled data acquired for the low resolution periphery isinterpolated to approximate data that would have been collected for theperiphery by X-ray detectors present in dead regions 150 of detectorarray 126. A “combined” data set of attenuation data comprising highresolution data acquired from HR-FOV 161 for the core of the imagingregion and interpolated low resolution data acquired from LR-FOV 162 forthe periphery of the imaging region is processed to provide an image ofthe imaging region and in particular an image of the core.

It is noted that for CT scanners in accordance with an embodiment of theinvention, having only one detector wing, i.e. wing 141 or wing 142,data collected by the one wing for the periphery at a given view anglecan be used to complete data acquired for the periphery at a view angleequal to the given angle plus 180°. Similarly, for CT scanners inaccordance with an embodiment of the invention, having complimentarywing configuration, i.e. wing configurations for which homologous areasare “negatives” of each other, data collected at a given view angle forthe periphery may be used to complete data acquired for the periphery ata view angle equal to the given angle plus 180°.

The inventor expects that an image of the core region can be generatedfrom the combined data set with a relatively small or negligentdegradation of image quality relative to quality of an image realizedfrom a data set comprising high resolution data for both the core andthe periphery. In this regard, the inventor notes that CT imagesacquired using a CT scanner with part of the scanner's X-ray beamshielded to reduce patient exposure to X-rays acquires relatively highresolution attenuation data for part of an imaging region of a patientand relatively low resolution attenuation data for a remainder of theimaging region. U.S. Pat. No. 6,385,278 referenced above describesgenerating a CT image of a patient's heart from data acquired using anX-ray beam shielded to reduce patient X-ray exposure. The data compriseshigh resolution attenuation data acquired for the heart region from anunshielded portion of the beam and data interpolated from low resolutionattenuation data acquired for tissue outside of the heart region fromshielded portions of the beam. The patent reports that the image of theheart has a resolution that approximates that of an image produced fromattenuation data comprising only high resolution data.

Often, a particular local region of interest LROI, for example the heartor liver, of a patient to be imaged by a CT scanner such as CT scanner120, in accordance with an embodiment of the invention, is notintersected by or located near to the patient's sagittal plane. Were thepatient to be scanned with scanner 120 and translated through FOV 152without properly aligning the patient with respect to high resolutionHR-FOV 161 of the FOV, the LROI would not necessarily be completelyincluded in a core region of the patient imaged with data from HR-FOV161. All the LROI volume might not pass through HR-FOV and a portionwould pass through LR-FOV 162. Therefore, in accordance with anembodiment of the invention, couch 46 in CT scanner 120 is moveablealong the x and y directions so that during translation of the patientthrough FOV 152 a desired LROI may be centered in HR-FOV 161.

FIGS. 3A and 3B schematically shows patent 122 being imaged by CTscanner 120, in accordance with an embodiment of the invention. By wayof example, in FIG. 3A an imaging region of patient 122 is an abdominalregion indicated by dashed lines 170 and an LROI of the patient is thepatient's liver 171. In FIG. 3A patient 122 is schematically shownbefore being aligned with the z-axis and the patient's liver isdisplaced from the z-axis along the negative x-axis. Were patient 122 tobe translated through FOV 152 without aligning the patient's liver withthe z-axis, a portion of the liver might not be imaged in highresolution HR-FOV 161 of FOV 152 and an image of the liver might nothave desirable resolution.

In accordance with an embodiment of the invention, the controller (notshown) of CT scanner 120 adjusts the position of couch 46 so that liver171 is substantially centered on the z-axis before and duringtranslation of patient 122 through FOV 152. As a result, during scanningof patient 122 with the scanner when imaging region 170 of the patientpasses through FOV 152, the liver is completely within HR-FOV 161 andhigh resolution data is acquired for the liver. FIG. 3B schematically,shows patient 22 passing through FOV 152 and the patient's livercentered in HR-FOV 161.

In accordance with an embodiment of the invention, to reduce exposure ofa patient to X-rays during scanning with a CT scanner in accordance withan embodiment of the invention, the scanner's cone beam is shielded tocollimate X-rays from the beam so that X-rays are provided substantiallyonly along directions that are incident on X-ray sensitive areas ofdetector array 126.

FIG. 4A schematically shows X-ray source 34 comprising shielding (notshown) to provide a collimated X-ray cone beam 138 suitable for use withdetector array 126 in accordance with an embodiment of the invention. Asa result of the collimation, cone beam 138 illuminates substantiallyonly detectors 132 in detector array 126 with X-rays and dead areas 150are not exposed to X-rays. Regions 139 of collimated cone beam 38 thatprovide X-rays that illuminate detectors 132 in array 126 are indicatedfor some detectors 132.

X-ray shielding configurations different from that used to provide conebeam 138 may be used, in accordance with an embodiment of the inventionto reduce a patient's exposure to X-rays. For example, shielding may beconfigured to shield and reduce X-ray intensity in a portion or portionsof a cone beam that illuminate a wing or wings of a detector array, suchas detector array 126 (FIG. 2A-FIG. 4) while, optionally, not affectinga portion of the X-ray beam that illuminates the high resolution region140 of the detector array. The shielding may by way of example take aform of a radiation filter, conventionally referred to as a “butterflyfilter”, having a smoothly varying thickness that increases withdisplacement to either side of focal spot 35. By way of another exampleuniform thickness “plate filters” may be used to attenuate X-rays thatilluminate the wings.

FIG. 4B schematically shows cone beam 38 shown in FIGS. 2A and 2B withdetector array 126 being filtered by two optionally uniform thicknessplate filters 181 and 182. Plate filters 161 and 162 filter sideportions 145 and 146 of cone beam 38 respectively but, optionally do notfilter central portion 144 of the beam. Optionally, plate filtersattenuate side portions 141 and 146 by a factor greater than 10.Optionally, the plate filters are mounted in addition to butterflyfilter. Optionally, the plate and butterfly filters attenuate the sideportions by a factor greater than 50. Optionally, the plate andbutterfly filters attenuate the side portions by a factor equal to about100.

For CT scanners in accordance with an embodiment of the invention havinga filtered beam, data acquired with filtered portions the beam iscalibrated using methods known in the art to data acquired fromunfiltered portions of the beam.

In the description and claims of the present application, each of theverbs, “comprise” “include” and “have”, and conjugates thereof, are usedto indicate that the object or objects of the verb are not necessarily acomprehensive listing of members, components, elements or parts of thesubject or subjects of the verb.

The present invention has been described using detailed descriptions ofembodiments thereof that are provided by way of example and are notintended to limit the scope of the invention. The described embodimentscomprise different features, not all of which are required in allembodiments of the invention. Some embodiments of the invention utilizeonly some of the features or possible combinations of the features.Variations of the described embodiments and embodiments of the inventioncomprising different combinations of features noted in the describedembodiments will occur to persons of the art. The scope of the inventionis limited only by the following claims.

1. A CT scanner comprising: a rotor rotatable about an axis of rotation;an X-ray source mounted to the rotor having a focal spot from which anX-ray beam emanates; an X-ray detector array comprising a plurality ofX-ray detectors for detecting X-rays in the X-ray beam; wherein thedetector array has at least one high resolution region in whichdetectors have a high packing density and at least one low resolutionregion in which detectors have a low packing density and are separatedby X-ray insensitive regions substantially larger than insensitiveregions resulting from septa between detectors that function to reducedetector cross talk.
 2. A CT scanner in accordance with claim 1, whereinthe at least one high resolution region comprises a single highresolution region.
 3. A CT scanner in accordance with claim 2, whereinthe high resolution region is flanked on one side by a single lowresolution area.
 4. A CT scanner in accordance with claim 2, wherein thehigh resolution region is flanked on both sides by a low resolutionregion.
 5. A CT scanner in accordance with claim 4, wherein the lowresolution regions have a same size.
 6. A CT scanner in accordance withclaim 5 wherein the detectors in the low resolution regions are arrayedin mirror image configurations.
 7. A CT scanner in accordance with claim4, wherein the detectors in the low resolution regions are arrayed sothat areas of one of the low resolution regions that have detectors arehomologous with areas of the other low resolution region that do nothave detectors.
 8. A CT scanner in accordance with claim 1, whereinadjacent detectors in a low resolution region of the detector array areseparated by a distance greater than 0.3 mm.
 9. A CT scanner inaccordance with claim 1, wherein adjacent detectors in a low resolutionregion of the detector array are separated by insensitive regions havingareas greater than or equal to about the size of the X-ray detectors.10. A CT scanner according to claim 1, wherein size of the detectors isa function of distance from a sagittal plane that passes through thefocal spot and axis of rotation and wherein detectors farther from thesagittal plane are larger than detectors closer to the plane. 11.(canceled)
 12. A CT scanner in accordance with claim 1, wherein theX-ray source and detector array define a field of view (FOV) for thescanner having a high resolution portion and a low resolution portion,wherein relatively high resolution attenuation data is acquired for aregion of a patient imaged by the CT scanner that passes through thehigh resolution portion and relatively low resolution attenuation datais acquired for a region of the patient that passes through the lowresolution region of the FOV.
 13. A CT scanner in accordance with claim12, wherein the high resolution region is defined by an area within afirst circle having a first diameter and the low resolution portion isdefined by an area between the first circle and a second circle having asecond diameter that is concentric with the first circle. 14.-20.(canceled)
 21. A CT scanner in accordance with claim 1 and comprisingX-ray radiation shielding that collimates X-rays from the X-ray sourceso that insensitive areas of the detector array are substantially notilluminated by X-rays.
 22. A CT scanner in accordance with claim 1comprising X-ray radiation shielding that attenuates X-rays provided bythe X-ray source and further comprising a processor that processesattenuation data acquired with the detector array and calibrates dataacquired responsive to X-rays attenuated by the shielding to dataacquired responsive to X-rays that are not attenuated by the shielding.23. A CT scanner in accordance with claim 1 and comprising a processorthat processes data for a region of a body imaged by the scanneracquired by the detector array low resolution region to estimate datathat would be acquired for the body region were the detector array lowresolution region to have a resolution substantially equal to that ofthe detector array high resolution region.
 24. A CT scanner inaccordance with claim 23, wherein the data acquired by the lowresolution region is processed using an interpolation algorithm.
 25. ACT scanner in accordance with claim 1, wherein at least some of thedetectors in the low resolution region are arrayed in incomplete rowsand/or columns.
 26. (canceled)
 27. A CT scanner in accordance with claim1, wherein the CT scanner is a multislice scanner.
 28. A CT scanner inaccordance with claim 1, wherein the detectors in the high resolutionand low resolution regions have substantially a same size.
 29. A CTscanner comprising: a rotor rotatable about an axis of rotation; anX-ray source mounted to the rotor having a focal spot from which anX-ray beam emanates; an X-ray detector array comprising a plurality ofX-ray detectors for detecting X-rays in the X-ray beam; wherein thedetector array has at least one high resolution region in whichdetectors have a high packing density and at least one low resolutionregion in which detectors have a low packing density and whereinadjacent detectors in a low resolution region of the detector array areseparated by insensitive regions having areas greater than or equal toabout the size of the X-ray detectors.
 30. A CT scanner in accordancewith claim 29, wherein the high resolution region is flanked on bothsides by a low resolution region.
 31. A CT scanner in accordance withclaim 30, wherein the detectors in the low resolution regions arearrayed in mirror image configurations.
 32. A CT scanner in accordancewith claim 30, wherein the detectors in the low resolution regions arearrayed so that areas of one of the low resolution regions that havedetectors are homologous with areas of the other low resolution regionthat do not have detectors.
 33. A CT scanner in accordance with claim29, wherein the detectors in the high resolution and low resolutionregions have substantially a same size.
 34. A CT scanner in accordancewith claim 29, wherein size of the detectors is a function of distancefrom a sagittal plane that passes through the focal spot and axis ofrotation and wherein detectors farther from the sagittal plane arelarger than detectors closer to the plane.
 35. A CT scanner inaccordance with claim 29 and comprising a processor that processes datafor a region of a body imaged by the scanner acquired by the detectorarray low resolution region to estimate data that would be acquired forthe body region were the detector array low resolution region to have aresolution substantially equal to that of the detector array highresolution region.
 36. A CT scanner in accordance with claim 29, whereinat least some of the detectors in the low resolution region are arrayedin incomplete rows and/or columns.
 37. A CT scanner comprising: a rotorrotatable about an axis of rotation; an X-ray source mounted to therotor having a focal spot from which an X-ray beam emanates; an X-raydetector array comprising a plurality of X-ray detectors for detectingX-rays in the X-ray beam; wherein the detector array has at least onehigh resolution region in which detectors have a high packing densityand at least one low resolution region in which detectors have a lowpacking density and wherein at least some of the detectors in the lowresolution region are arrayed in incomplete rows and/or columns.
 38. ACT scanner in accordance with claim 37, wherein the high resolutionregion is flanked on both sides by a low resolution region.
 39. A CTscanner in accordance with claim 37, wherein the detectors in the lowresolution regions are arrayed in mirror image configurations.
 40. A CTscanner in accordance with claim 37, wherein the detectors in the lowresolution regions are arrayed so that areas of one of the lowresolution regions that have detectors are homologous with areas of theother low resolution region that do not have detectors.
 41. A CT scannerin accordance with claim 37, wherein size of the detectors is a functionof distance from a sagittal plane that passes through the focal spot andaxis of rotation and wherein detectors farther from the sagittal planeare larger than detectors closer to the plane.
 42. A CT scanner inaccordance with claim 37, and comprising a processor that processes datafor a region of a body imaged by the scanner acquired by the detectorarray low resolution region to estimate data that would be acquired forthe body region were the detector array low resolution region to have aresolution substantially equal to that of the detector array highresolution region.
 43. A CT scanner comprising: a rotor rotatable aboutan axis of rotation; an X-ray source mounted to the rotor having a focalspot from which an X-ray beam emanates; an X-ray detector arraycomprising a plurality of X-ray detectors for detecting X-rays in theX-ray beam that have been attenuated by an imaged body, wherein thedetector array has at least one high resolution region in whichdetectors have a high packing density and at least one low resolutionregion in which detectors have a low packing density; and a processoradapted to reconstruct images for a region of a body covered by thedetector array high resolution region; wherein said reconstructionincludes processing of data for a region of a body covered by thedetector array low resolution region to estimate data that would beacquired for the body region were the detector array low resolutionregion to have a resolution substantially equal to that of the detectorarray high resolution region.
 44. A CT scanner in accordance with claim43, wherein the high resolution region is flanked on both sides by a lowresolution region.
 45. A CT scanner in accordance with claim 44, whereinthe detectors in the low resolution regions are arrayed in mirror imageconfigurations.
 46. A CT scanner in accordance with claim 43, whereinthe detectors in the low resolution regions are arrayed so that areas ofone of the low resolution regions that have detectors are homologouswith areas of the other low resolution region that do not havedetectors.
 47. A CT scanner in accordance with claim 43, wherein size ofthe detectors is a function of distance from a sagittal plane thatpasses through the focal spot and axis of rotation and wherein detectorsfarther from the sagittal plane are larger than detectors closer to theplane.
 48. A CT scanner in accordance with claim 43, wherein the atleast one high resolution region comprises a single high resolutionregion.
 49. A CT scanner in accordance with claim 43, wherein the highresolution region is flanked on one side by a single low resolutionarea.
 50. A CT scanner in accordance with claim 43, wherein adjacentdetectors in a low resolution region of the detector array are separatedby a distance greater than 0.3 mm.
 51. A CT scanner in accordance withclaim 43, wherein the CT scanner is a multislice scanner.
 52. A CTscanner in accordance with claim 43, wherein the detectors in the highresolution and low resolution regions have substantially a same size.53. A CT scanner in accordance with claim 43, wherein the X-ray sourceand detector array define a field of view (FOV) for the scanner having ahigh resolution portion and a low resolution portion, wherein relativelyhigh resolution attenuation data is acquired for a region of a patientimaged by the CT scanner that passes through the high resolution portionand relatively low resolution attenuation data is acquired for a regionof the patient that passes through the low resolution region of the FOV.54. A CT scanner in accordance with claim 53, wherein the highresolution region is defined by an area within a first circle having afirst diameter and the low resolution portion is defined by an areabetween the first circle and a second circle having a second diameterthat is concentric with the first circle.
 55. A CT scanner in accordancewith claim 43, and comprising X-ray radiation shielding that collimatesX-rays from the X-ray source so that insensitive areas of the detectorarray are substantially not illuminated by X-rays.
 56. A CT scannercomprising: a rotor rotatable about an axis of rotation; an X-ray sourcemounted to the rotor having a focal spot from which an X-ray beamemanates; an X-ray detector array comprising a plurality of X-raydetectors for detecting X-rays in the X-ray beam that have beenattenuated by an imaged body, wherein the detector array has at leastone high resolution region in which detectors have a high packingdensity and at least one low resolution region in which detectors have alow packing density; X-ray radiation shielding that attenuates X-raysprovided by the X-ray source adapted to attenuate the X rays impingingon the at least one low resolution region more than the attenuation ofX-Rays impinging on the at least one high resolution region; and aprocessor that processes attenuation data acquired with the detectorarray and calibrates data acquired responsive to X-rays attenuated bythe shielding in the at least one low resolution region to data acquiredresponsive to X-rays in the high resolution region.
 57. A CT scanner inaccordance with claim 56, wherein said X-ray radiation shieldingattenuates the X rays impinging on the at least one low resolutionregion by at least a factor of 10 compared to the intensity of X-Raysimpinging on the at least one high resolution region.
 58. A CT scannerin accordance with claim 56, wherein said X-ray radiation shieldingattenuates the X rays impinging on the at least one low resolutionregion by at least a factor of 50 compared to the intensity of X-Raysimpinging on the at least one high resolution region.
 59. A CT scannerin accordance with claim 56, wherein the high resolution region isflanked on both sides by a low resolution region.
 60. A CT scanner inaccordance with claim 59, wherein the detectors in the low resolutionregions are arrayed in mirror image configurations.
 61. A CT scanner inaccordance with claim 59, wherein the detectors in the low resolutionregions are arrayed so that areas of one of the low resolution regionsthat have detectors are homologous with areas of the other lowresolution region that do not have detectors.
 62. A CT scanner inaccordance with claim 56, wherein size of the detectors is a function ofdistance from a sagittal plane that passes through the focal spot andaxis of rotation and wherein detectors farther from the sagittal planeare larger than detectors closer to the plane.
 63. A CT scanner inaccordance with claim 56, wherein the at least one high resolutionregion comprises a single high resolution region.
 64. A CT scanner inaccordance with claim 56 wherein the high resolution region is flankedon one side by a single low resolution area.
 65. A CT scanner inaccordance with claim 56, wherein adjacent detectors in a low resolutionregion of the detector array are separated by a distance greater than0.3 mm.
 66. A CT scanner in accordance with claim 56, wherein thedetectors in the high resolution and low resolution regions havesubstantially a same size.
 67. A CT scanner in accordance with claim 56,wherein the X-ray source and detector array define a field of view (FOV)for the scanner having a high resolution portion and a low resolutionportion, wherein relatively high resolution attenuation data is acquiredfor a region of a patient imaged by the CT scanner that passes throughthe high resolution portion and relatively low resolution attenuationdata is acquired for a region of the patient that passes through the lowresolution region of the FOV.
 68. A CT scanner in accordance with claim67, wherein the high resolution region is defined by an area within afirst circle having a first diameter and the low resolution portion isdefined by an area between the first circle and a second circle having asecond diameter that is concentric with the first circle.
 69. A CTscanner in accordance with claim 56, and comprising X-ray radiationshielding that collimates X-rays from the X-ray source so thatinsensitive areas of the detector array are substantially notilluminated by X-rays.
 70. A CT scanner comprising: a rotor rotatableabout an axis of rotation; an X-ray source mounted to the rotor having afocal spot from which an X-ray beam emanates; an X-ray detector arraycomprising a plurality of X-ray detectors for detecting X-rays in theX-ray beam that have been attenuated by an imaged body, wherein thedetector array has at least one high resolution region in whichdetectors have a high packing density and at least one low resolutionregion in which detectors have a low packing density; X-ray radiationshielding that attenuates X-rays provided by the X-ray source adapted toattenuate the X rays impinging on the at least one low resolution regionmore than the attenuation of X-Rays impinging on the at least one highresolution region by a factor of at least
 10. 71. A CT scanner inaccordance with claim 70, wherein said X-ray radiation shieldingattenuates the X rays impinging on the at least one low resolutionregion by at least a factor of 50 compared to the intensity of X-Raysimpinging on the at least one high resolution region.